1. Field of the Invention
The present invention relates to a carbon fiber having a novel structure providing a high strength and high modulus obtained by using a mesophase pitch as the starting material, and a precursor thereof. More specifically, it pertains to a high performance pitch based carbon fiber having a high strength and high modulus, particularly a improved tensile strength, and having a modified fiber surface structure, and a precursor thereof, i.e., a pre-carbonized fiber.
2. Description of the Related Art
Carbon fibers have a high specific strength and specific modulus and have attracted attention as strong and light materials for use in the fields of space craft, automobiles, and other industries. In such fields, there is a need for inexpensive materials having a high strength and high modulus.
Known carbon fibers include PAN based carbon fibers obtained by using polyacrylonitrile (PAN) as the starting material and pitch based carbon fibers obtained by using pitch as the starting material, but currently, the PAN based carbon fibers are primarily used as high performance carbon fibers having a high strength and high modulus.
PAN based carbon fibers, however, should be subjected to heat-treatment processes to achieve a very high modulus, The PAN used as the starting material therefor is expensive, and the carbon yield of PAN is low, thus the PAN based carbon fiber is inevitably high.
Accordingly, there has been proposed a carbon fiber having a three-dimensional polycrystalline graphite structure order, which is disclosed in U.S. Pat. No. 4,005,183. The carbon fiber is obtained by using a mesophase pitch as the starting material at a very high carbon fiber yield and a easily has a very high modulus.
Recently, improved pitch based carbon fiber and their structures having an even higher strength have been proposed. For example, there have been proposed a structure in which the graphite crystals in the fiber cross-sectional direction are made finer by stirring the portion immediately above the capillary portion of spinning nozzles, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-104927; the structure in which the carbon layer surface in the fiber cross-sectional direction is folded, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-41320; or the structure in which a lattice pattern is formed on the fiber cross-section by placing a metal net in front of the nozzle, as disclosed in U.S. Pat. No. 4,818,612. These structures are all produced by modifying the process, and the graphite crystals constituting the fiber are made finer, and the fiber becomes to be less graphitizeable in the cross-sectional direction.
As the method of obtaining a novel fiber structure by modifying a infusibilization or carbonization process, or by a combination of both, a fiber structure is known in which the graphite crystallizability internally of the fiber is improved by a selective infusibilization of the outer surface layer portion of the fiber, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-120112. This fiber structure has a specific feature in that a high modulus can be easily obtained.
As the preparation method, Japanese Unexamined Patent Publication (Kokai) No. 1-314733 discloses a method of obtaining a high strength by incorporating iodine with an atmospheric gas during the infusibilization process. This publication states that this method can be used to prepare a fiber with substantially no defects, by suppressing the oxygen introduced into the fiber during the infusibilization to a minimum value.
Japanese Unexamined Patent Publication (Kokai) No. 60-259629 discloses a method of preparing graphitized fibers having the advantages of a shortened infusibilization and carbonization time, and a high strength, by graphitizing the fusibilized fiber infusibilized with nitrogen dioxide at a temperature elevation speed of 40.degree. C./min or higher. In a fiber disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-259629, a satisfactory physical property can not be obtained by passing same through the pre-carbonized fiber stage, as described hereinbelow. The high strength carbon fiber of the present invention naturally derived from the fiber structure obtained at the pre-carbonized fiber stage is different from that of the fiber disclosed in! Japanese Unexamined Patent Publication No, 60-259629.
Japanese Unexamined Patent Publication (Kokai) No. 61-215716 discloses a method of improving the tensile strength by a gas phase oxidation of the pitch based carbon fiber surface under specific conditions. This method is considered to improve the carbon fiber's physical properties by etching the carbon fiber surface to remove defects thereon.
Japanese Unexamined Patent Publication (Kokai) No. 61-225330 discloses a high strength PAN based carbon fiber having a structure in which the surface layer portion has substantially the same perfection as a crystal composing the fiber center portion, and a less perfection of an ultra-thin outermost layer portion. This fiber structure is obtained by subjecting the PAN type carbon fibers to an electrochemical oxidation treatment under specific conditions, and then inactivating same in an inert or reducing atmosphere.
As a result of the investigations by the present inventors it was found that, even when this method is applied to the mesophase pitch based carbon fibers to obtain a similar fiber structure, the tensile strength not improved, but instead, was lowered. This was considered to be due to the great difference in the fiber structure of the PAN based carbon fibers and that of the pitch based carbon fibers, as can be seen from the crystallite sizes.
Therefore, for the pitch based carbon fibers having a high strength as proposed in the prior art, there have been only proposed a structure in which the crystals in the fiber cross-sectional direction were made finer, or a conventional strength improvement method of reducing various defects occurring during preparation of fibers, as much as possible, and a novel carbon fiber structure having a high strength and high modulus and retaining the inherent characteristics of the pitch type carbon fiber has not been proposed.
Potential defects occurring in the steps of preparing the carbon fibers, or surface defects occurring during the handling after the preparation of the carbon fibers will greatly lower the strength of carbon fibers.